Delicate backroom negotiations over legislation for a massive Wall Street rescue were jolted yesterday by Sen. John McCain's sudden announcement that he would come to Washington to participate in the talks. But by day's end, Sen. Barack Obama's decision to join his rival at the White House in a show of bipartisanship appeared to set the stage for a final deal.

Democratic congressional leaders had been demanding for days that McCain take a more forceful position in favor of a bailout and rally his party behind him. What they did not want was either McCain or Obama in the room -- a prospect that suddenly seemed to materialize yesterday, only to dissipate in a rapid-fire series of events. Instead, negotiators now appear to have gotten more than they could have hoped for: both presidential candidates meeting with President Bush to deliver a forceful statement in favor of a fast, dramatic market intervention.

"The meeting itself will be short," said a Democrat familiar with the plans. "The most important message will be [Obama] and McCain understand the need to unite to get this done."

McCain's proposal yesterday afternoon to suspend campaigning and involve both him and Obama in negotiations -- dismissed as "erratic," grandstanding and even "somewhere on the stupidity scale between plain silly and numbingly desperate" by former Republican congressman Mickey Edwards (Okla.) -- actually set into motion what may prove to be the push negotiators needed.

Obama had called McCain earlier in the day to propose a joint statement on the bailout. By last night, that had turned into an unexpected summit meeting. Until Obama agreed to come to Washington, Democrats had been furious and GOP leaders were wary of their nominee's unilateral decision when an agreement seemed close without intervention from the candidates. House Minority Leader John A. Boehner (R-Ohio) joined House Speaker Nancy Pelosi (D-Calif.) in issuing a statement saying that "working in a bipartisan manner, we have made progress" -- without McCain.

"I really don't think we can give him a seat at the table unless Barack Obama is willing to join him," House Minority Whip Roy Blunt (R-Mo.) said before Obama's announcement.

The negotiating table has thus far been extremely small. The talks have been led by House Financial Services Committee Chairman Barney Frank (D-Mass.); Rep. Spencer Bachus (Ala.), the committee's ranking Republican; and Treasury Secretary Henry M. Paulson Jr. and his staff. Senate banking committee Chairman Christopher J. Dodd (D-Conn.) and his ranking Republican, Sen. Richard C. Shelby (Ala.), have had a more limited role, while congressional leaders have focused on bringing their members around simply to the concept of a $700 billion bailout of teetering financial firms.

Negotiators questioned how either candidate could enter those talks, especially when a process that started badly on Tuesday seemed close to success yesterday. It now appears neither McCain nor Obama will join those discussions, congressional aides said.

"Obama and McCain coming back will not be particularly helpful," said House Majority Leader Steny H. Hoyer (D-Md.). "It's just going to bring the presidential campaign into the halls of Congress directly."

Until yesterday, the greatest concern among Democrats was that McCain could walk away from the bailout effort, attacking it as a rescue plan for Wall Street millionaires and leaving Democratic leaders lined up with an unpopular president and an unpopular package.

Until last night, Bush had also has been largely absent from the crisis as well. But Republican aides said that it is their "standard-bearer" -- not their highly unpopular commander in chief -- who could rally wavering Republicans behind a final deal, and that the senator from Arizona was reacting to Democratic argument that a true bipartisan deal depends on his support.

Why, McCain aides asked, would the Republican back the plan without having shaped it?